1. Field of the Invention
The present invention relates to a constant current switching power supply apparatus (also called “switched-mode power supply apparatus”) which can shorten the rising time of a pulsative current, in particular a relatively large pulsative current, which is supplied from a switching power supply to a load. The invention also relates to a light source driving apparatus for driving a light source emitting light responsive to a pulsative current, and an image display apparatus displaying an image by having a light source emit light pulsatively. The invention also relates to a method driving the switching power supply apparatus, and a method of driving the light source driving apparatus.
2. Description of Related Art
A switching power supply apparatus is known as a small-sized power supply apparatus having a high efficiency, and is used as a power supply for electronic equipment in various devices. It is often used as a constant voltage power supply for supplying a constant voltage to a load, but it is also possible to use it as a constant current power supply for supplying a constant current to a load. In recent years, such a switching power supply has begun to be used as a constant current power supply for supplying a pulsative constant current. A typical such application is described below.
A commercially available projection television set uses, as a light source, any of various lamps (discharge-type Xenon lamp, metal-halide lamp, halogen lamp or other white-light lamps), and as a light valve, a liquid crystal, DMD (digital micromirror device), or other spatial modulation device. in addition, projection television sets which use a light emitting diode (LED) or a semiconductor laser diode (LD) as the light source, in an attempt to extend the life time of the light source and to expand the color gamut, are becoming to be realized. In such projection television sets, a lamp, LED, or LD is driven basically by a constant current, although there are some variations with regard to the waveforms of the driving currents. By driving with a constant current, the light intensity can be stabilized with respect to the time for which the light source has been in use.
In particular, in a projection television set using LEDs or LDs as a light source, three elements respectively emitting monochromatic light of red, green and blue, are driven so that they emit light on a time division basis at a very high frequency (several hundred Hz or several kilo Hz) so that the switching is not perceived by human eyes, as for instance, disclosed in Japanese Patent Publication, No. 2007-65677 (page 13, and FIG. 16). In order for the light emitting elements of the respective colors to emit light stably, pulsative constant currents have to be supplied on a time division basis to the respective light emitting elements.
The luminance on the screen of a projection television set is increased with increase in the product of the instantaneous light intensity of the light source (which depends on the driving current of the light emitting element) and the length of time of light emission in each time slot of the time division. Accordingly, for a given time of light emission, the rise time and fall time of the puslative current need to be shortened in order to increase the luminance.
In a known drive apparatus for an LED light device, a switching power supply is used. In this case, a switching element is connected, as a light intensity adjusting means, in series with an LED, and by turning on and off the switching element, a pulsative current is made to flow, as disclosed, for instance, in Japanese Patent Publication No. 2005-142137 (page 8, FIG. 11).
Thus, it is possible to shorten the fall time of the pulsative current by connecting a switching element in series with the LED and turning off the switching element. The rise time is not problematical when the current is small (e.g., when it is not more than 1A). When, however, the current value is relatively large (10A or more), the response time of the drive apparatus is unacceptably long, and the rise time is unacceptably long.